Similar considerations apply to the situation of a person rotating the leg over a fixed foot. Any rotation imparted to the tibia is transmitted to the subtalar joint (Fig. 16-31). For example, if one rotates the leg externally over a foot that is fixed to the ground, the subtalar joint undergoes a movement of supination. This is analogous to movement about a mitered hinge; movement of one component about a vertical axis is transmitted to the second component as movement about a horizontal axis. Supination causes the calcaneus to assume a varus position, which, because the metatarsals remain flat on the ground, increases the twist in the foot and raises the arch.25 The opposite occurs with internal tibial rotation; pronation of the hindfoot causes a relative supination of the forefoot; the foot untwists and the arch flattens. This can easily be observed if one attempts to rotate the leg with the foot fixed to the ground. With respect to the structural alignment, then, a person with excessive internal tibial torsion will tend to have a pronated hindfoot (calcaneus in valgus position) and a forefoot that is supinated with respect to the hindfoot. The resultant untwisting of the foot causes a flatfoot on standing. The person with excessive external tibial torsion will tend to have a varus heel and a high arch.